The present invention generally relates to a semiconductor integrated circuit device, and more particularly to a semiconductor integrated circuit device having a circuit which generates an internal clock signal used for outputting data to an external circuit through an output buffer.
Generally, data generated in a large scale integrated circuit device (hereafter simply referred to as an LSI device) such as a microcomputer is output to an external circuit through a built-in output buffer which is controlled by an internal clock signal generated in the LSI device.
Most of LSI devices such as a direct memory access controller have an internal clock generator, which generates internal clock signals from an external clock signal supplied thereto from an external device. The generated internal clock signals are used to define timing of operation in an internal logic circuit and input and output buffers which interface with external devices. Generally, internal clock generators generate two internal clock signals which do not overlap each other in view of timing. In order to generate the internal clock signals from the external clock signal, an internal clock generator includes a flip-flop and a delay line. Precision in timing of the internal clock signals depends on precision in the delay line.
Generally, one of the generated internal clock signals is used for defining operation timing of an output buffer. Therefore, the operation timing of the output buffer depends on precision in the delay line provided in the internal clock generator. It is noted that it is very difficult to form a stabilized delay line so as to always provide a fixed delay amount. In other words, actual delay lines have variations in electric characteristics. It follows that operation of the output buffer driven by the internal clock signal thus generated is unstable, and therefore the reliability thereof is not high. Additionally, with the above-mentioned structure, it is very difficult to obtain satisfactory alternating current characteristics (hereafter simply referred to as AC characteristics) with respect to signal transfer characteristics and the like.